I'm not quite sure what makes a song "classic" to me, basically they just sound like it, but these are more or less what I consider to be Maiden's classics:
"Prowler", "Remember Tomorrow", "Running Free", "Phantom of the Opera", "Transylvania", "Charlotte the Harlot", "Iron Maiden", "Sanctuary", "The Ides of March", "Wrathchild", "Murders In the Rue Morgue", "Genghis Khan", "Killers", "Invaders", "Children of the Damned", "The Prisoner", "22 Acacia Avenue", "The Number of the Beast", "Run to the Hills", "Hallowed Be Thy Name", "Where Eagles Dare", "Revelations", "Flight of Icarus", "Die With Your Boots On", "The Trooper", "Still Life", "Aces High", "2 Minutes To Midnight", "Losfer Words (Big 'Orra)", "Flash of the Blade", "The Duellists", "Back In the Village", "Powerslave", "Rime of the Ancient Mariner", "Caught Somewhere In Time", "Wasted Years", "Sea of Madness", "Heaven Can Wait", "Stranger In a Strange Land", "Alexander the Great", "Moonchild", "Infinite Dreams", "Can I Play With Madness", "The Evil That Men Do", "Seventh Son of a Seventh Son", "The Clairvoyant", "Tailgunner", "Holy Smoke", "No Prayer For the Dying", "Bring Your Daughter...To the Slaughter", "Be Quick Or Be Dead", "From Here To Eternity", "Afraid To Shoot Strangers", "Judas Be My Guide", "Fear of the Dark", "Sign of the Cross", "Lord of the Flies", "Man On the Edge", "The Edge of Darkness", "Futureal" and "The Clansman".
Then on to the modern classics:
"The Wicker Man", "Ghost of the Navigator", "Brave New World", "Blood Brothers", "The Mercenary", "Dream of Mirrors", "The Fallen Angel", "Wildest Dreams", "Rainmaker", "No More Lies", "Dance of Death", "Paschendale", "Face In the Sand", "Journeyman", "Different World", "These Colours Don't Run", "Brighter Than a Thousand Suns", "The Pilgrim", "The Longest Day", "Out of the Shadows", "The Reincarnation of Benjamin Breeg", "For the Greater Good of God", "Lord of Light", "The Legacy", "Satellite 15...The Final Frontier", "El Dorado", "Mother of Mercy", "Coming Home", "The Alchemist", "Isle of Avalon", "Starblind", "The Talisman", "When the Wild Wind Blows", "If Eternity Should Fail", "Speed of Light", "The Great Unknown", "The Red and the Black", "The Book of Souls", "Death or Glory", "Shadows of the Valley", "Tears of a Clown" and finally "Empire of the Clouds".